The present invention relates generally to nuclear reactors and, more particularly, is directed to an improved control rod for use with a nuclear fuel assembly in a pressurized water reactor.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. Conventional designs of these fuel assemblies include a plurality of fuel rods and hollow tubes or guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the guide thimbles in thereby forming an integral fuel assembly. Generally, in most reactors, a fluid coolant such as water, is directed upwardly through aperatures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom.
One method for controlling the reactivity within the core is through the employment of neutron absorbing elements or rods, commonly referred to as "control rods". One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919; granted to Donald J. Hill. Shown is a fixed array of control rods supported at their upper ends by a spider assembly, which in turn, is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of a control rod used in such an arrangement is in the form of an elongated metallic tube or cladding having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. Generally, the neutron absorbing material is in the form of a stack of closely packed ceramic or metal pellets which only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug in defining a plenum chamber for receiving gasses generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rod.
The inner diameter of the guide tubes is usually chosen to be the maximum permitted by the fuel assembly lattice in order that the maximum possible diameter control rod can be inserted therein. It is desirable to maximize the diameter of the absorber pellets in the control rod because the absorption effectiveness of the rods is very strongly dependent, particularly in thermal neutron reactors, on the surface area of the pellets. For this reason, and to promote heat transfer, there usually are narrow clearances between the absorber pellets and the control rod clad (cladding), and between the control rod and its guide tube.
The normal operation of the control rods usually consists of several steps of the drive mechanism at small increments, such as at 0.6 inch per step. When the control rod is moved, there is a step increase (abrupt change in neutron flux) in power in the pellets adjacent the end of the rod. This increase in pellet power results in thermal expansion of the pellets into the clad, which causes a relatively high tensile strain rate in the clad. This, along with chemical attack on the inside of the pellet causes pellet clad interaction (PCI). PCI can damage or destroy the effectiveness of the clad or even cause a rod to rupture; all of which equates to premature rod failure and a shortened useful life thereof.
Early failure of a control rod caused from the neutron flux density was recognized in U.S. Pat. No. 4,172,762, granted to Anthony et al. Anthony et al solution to the problem was the design of a control rod wherein the pellets in the lower end of the rod have a radius smaller than that of the other pellets and are wrapped with a sleeve having a linear compressibility sufficient to accomodate exposure-induced radial expansion of the end pellets without producing excessive clad strain. They suggest the sleeve be a sponge material made from type 347 stainless steel at 22.5% theoretical density to accomplish their objectives. In order to do this, the (boron carbide) pellets must have a smaller diameter at the tip of the rod so that the cylindrical sponge sleeve can be inserted between the clad (inside diameter) and the boron carbide pellets (outside diameter). Although such design may have its benefits, the requirement that some pellets have a smaller diameter than others and the addition of an extra sleeve not only adds material costs, but also, increases the manufacturing steps and requires more time which results in a more expensive control rod.
Although Hitchcock, in U.S. Pat. Nos. 3,230,147 and 3,255,086, did not specifically recognize the pellet-clad-interaction (PCI) problem, he set forth two different control rod designs, which when considered from a nuclear standpoint, can be considered to be similar to a movable burnable poison rod in that they permit control of the radial peaking factors. In one design, the rod is cone-shaped or tapered from one end to the other in creating a graduated form of neutron absorbing capacity so that movement of the rod can take place without creating significant zones of stepwise change of reactivity in the nuclear reactor core structure. In the other design, the rod is made in a series of four parts with the neutron absorbing capacity of the parts increasing from one part to the next so as to create the tapered effect. In these two designs, the basic concept is to have a control rod having a length which is twice the core height to enable the rod to either be inserted through the bottom of the core or withdrawn from the top of the core. Present day pressurized water reactors (PWRs) are not designed for control rods projecting below the bottom core plate of the reactor. Further, in Hitchcock, the lower parts of the rod are made of varying absorber materials (thin-walled and thick-walled boron and mild steel) with the upper part filled with boron graphite. In practice, the boron steel and mild steel will become embrittled in a short time so the design is not realistic from a materials standpoint. Still further, the lower parts are filled with water which makes them perform as "flux trap" devices. Besides the above disadvantages, such rod designs are complex in structure and expensive to manufacture.
Consequently, a need still exists for a control rod adapted to be used with a pressurized water reactor which is not only simple in structure and thus less costly to manufacture but, in addition thereto, a control rod so designed to alleviate pellet-clad-interaction (PCI) and thereby prolong the useful life of a rod.